1. Field of the Invention
The present invention relates to a finder apparatus and an imaging apparatus. In particular, the present invention relates to a finder apparatus that superimposes, on an image formed by an observation optical system of a reverse-Galilean finder, display of a field-of-view frame or the like for example to make a user visually recognize the image on which such display is superimposed. Further, the present invention relates to an imaging apparatus in which the finder apparatus is mounted.
2. Description of the Related Art
Conventionally, reverse-Galilean finders were used in finder apparatuses of cameras or the like (please refer to Japanese Unexamined Patent Publication No. 2000-352740 (Patent Document 1), Japanese Unexamined Patent Publication No. 2001-013549 (Patent Document 2), Japanese Unexamined Patent Publication No. 7(1995)-218987 (Patent Document 3), and Japanese Patent No. 3026233 (Patent Document 4)). The reverse-Galilean finder is composed of an objective lens having negative refractive power and an eyepiece lens having positive refractive power. In some finder apparatuses, information such as a field-of-view frame is displayed by using a display member, and a reflective member such as a half mirror is provided in an optical path of the observation optical system to make a user observe an image in which the information such as the field-of-view frame is superimposed on a subject image formed by the observation optical system. Among such finder apparatuses, a finder apparatus that illuminates information, such as a field-of-view frame, by using external light is called as an illuminating-window-type finder.
In many finder apparatuses that superimpose information about a field-of-view frame or the like on an image formed by an observation optical system to make a user observe such an image, an optical system for guiding images to the observer's eye, which is called as a target lens, is provided between a display member and an eyepiece lens. The target lens is provided to make the information, such as the field-of-view frame, and the diopter scale of a subject image become appropriate for the observer's eye, and to make the field of view in actual observation coincide with a range in which the field-of-view frame is displayed.
In the illuminating-window-type finder, information such as a field-of-view frame is illuminated by external light. Therefore, when the illumination intensity of the external light is low, it is difficult to recognize the information. Especially, when no external light is available at night, it is impossible to use the information. Therefore, use of an additional illumination apparatus or a light emitting member, such as an LED, has been proposed. In recent years, finder apparatuses using, as display members, liquid crystal devices with backlight illumination gradually increased. In the field of such finder apparatuses using liquid crystal devices, a remarkable development is seen in so-called electronic viewfinders. The electronic viewfinders do not simply display a field-of-view frame, a target mark or the like, but can display also an electronic image of a subject image obtained by the imaging lens. Therefore, there is a growing demand for optical systems that can cope with electronic viewfinders.
Further, smaller liquid crystal devices than conventional devices were developed in recent years, and a reduction in the size of finders became requested as the size of apparatuses became smaller. Therefore, the size of display members and the size of display on the display members tend to become smaller, compared with conventional display members. Meanwhile, the angles of imaging lens systems became wider. Therefore, when information such as the field-of-view frame displayed in small size is superimposed on the subject image, the image of the field-of-view frame or the like needs to be magnified. Hence, the target lens needs to have extremely strong refractive power. Consequently, sufficient correction of aberrations becomes difficult, and it becomes more difficult to make the diopter scale of a central area of an image and the diopter scale of a peripheral area of the image uniform, compared with conventional finders. Further, in many conventional finder apparatuses, only a field-of-view frame and a target mark for measuring distance were displayed. Therefore, optical systems that pay attention only to the central area of the image and the peripheral area of the image were sufficient. However, in the electronic viewfinders, an image obtained by imaging is also displayed. Therefore, it is necessary that the optical system can provide excellent diopter scale conditions continuously from the central area of the image through the peripheral area of the image.
However, neither of Patent Document 1 and 2 considers a difference in diopter scale between the central area of the image and the peripheral area of the image. In Patent Document 3, a central area of an optical image frame, which is a display member, and a peripheral area of the optical image frame are arranged at different positions from each other along an optical axis to obtain an optical image, such as a field-of-view frame, in excellent diopter scale conditions. In Patent Document 4, at least one of surfaces of the target lens and the eyepiece lens is an aspheric surface, and the shape of the aspheric surface is regulated to provide an excellent view of the optical image frame.
However, in the technique disclosed in Patent Document 3, a display member that is curved based on the diopter scale conditions is necessary to obtain excellent diopter scale conditions continuously from the central area of the image through the peripheral area of the image. Therefore, it would be difficult to apply such a curved display member to an electronic viewfinder, the size of which became smaller in recent years. Further, the apparatus disclosed in Patent Document 4 assumes only a central area of the image, in which a distance measurement area is displayed, and a peripheral area of the image, in which an optical image frame is displayed, in an illuminating-window-type finder. Therefore, it is impossible to obtain excellent diopter scale conditions continuously from the central area of the image through the peripheral area of the image.